In some cases it can be really confusing if it's a friend(s) joining and playing on your computer because they don't have internet. Like in my case. I have 3 friends with no internet so they often come to my house to use it. I or they do announce it on different sites they join that I'm also on though.--

And yes it's through IP addresses. They go to the list of users and click on one. Then they can see the IP number and track from which country/town/area they're using the internet from. It's kinda stalker but almost every site owner does that whenever they want to ban someone permanently from their site. And that means that they can't even make new accounts. o3o